pgsql: Rely on plan_cache_mode to force generic plans in partition_prun

Rely on plan_cache_mode to force generic plans in partition_prune test.

This file had a very weird mix of tests that did "set plan_cache_mode =
force_generic_plan" to get a generic plan, and tests that relied on
using five dummy executions of a prepared statement. Converting them
all to rely on plan_cache_mode is more consistent and shaves off a
noticeable fraction of the test script's runtime.

Discussion: https://postgr.es/m/11952.1569536725@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d52eaa094847d395f942827a6f413904e516994c

Modified Files
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src/test/regress/expected/partition_prune.out | 295 +-------------------------
src/test/regress/sql/partition_prune.sql | 82 +------
2 files changed, 6 insertions(+), 371 deletions(-)